Ebonite is the product of the vulcanization reaction of synthetic or natural rubber (high molecular weight cis-1,4-polyisoprene) with an excess of sulfur. Typically, the rubber vulcanization process takes place when 0.5-5 parts (by weight) of sulfur is combined under reactive conditions with 100 parts of rubber. A cross-linked network polymer structure results in which sulfur bridges link the polymeric natural rubber chains.
If vulcanization is allowed to continue until considerably more sulfur has combined with the natural rubber, a rigid, nonelastomeric plastic known as hard rubber or ebonite is formed. Ebonite is a hard, inextensible solid containing about 30-50% combined sulfur. Ebonite is long-known to the art, having been first made in the 1840s and produced on a commercial scale since about 1860, e.g., see U.S. Pat. Nos. 22,218, 46,609, 48,992, 48,993, 53,643 and 76,293.
Ebonite can be readily machined and is often produced in bar, tube or sheet stock for this purpose. Its main uses exploit its chemical inertness and corrosion resistance and its electrical and thermal insulating properties. However, the material softens at about 50.degree. C., therefore, it is not suitable for high-temperature applications. Moreover, it is difficult, if not impossible, to apply sections of bar, tube or sheet ebonite stock to small parts or to parts with complicated shapes or profiles, for example, by gluing. Furthermore, it is difficult to form thin coatings from ebonite. Therefore, a rubber product with good chemical inertness and corrosion resistance and that is easily applied, e.g., by coating, onto such small and/or intricately shaped parts is highly desirable.
The present invention, a liquid ebonite mixture or LEM, satisfies these requirements. For the main rubber component, LEM comprises not a conventional solid rubber but a liquid rubber, which will be described in greater detail below. Liquid ebonite mixture, having excellent abrasion resistance and chemical resistance, is ideally suited for coatings, such as monolithic thick-layer coatings. LEM may be used to coat such small and/or intricately-shaped articles as (1) rotors of centrifuges for filtration processes and the working wheels of pumps and fans, (2) the inside surfaces of pipelines, fittings, etc. of small diameter, i.e., less than about 32 mm inner diameter, (3) perforated elements such as screens and mesh, (4) very intricate surface parts, e.g. membrane equipment and membrane hydrolysis apparatus, (5) chemical apparatus, reactor vessels and complex air ducts up to 500 mm in diameter, such as those with joining lips, and (6) galvanic or electrolysis baths and their components. Coating with LEM eliminates the need for an additional glue layer and provides adhesion strength to steel up to 11 MPa in tear-apart tests.
U.S. Pat. No. 4,195,009 discloses coating systems comprising liquid rubber. However, the liquid rubber must be present in the form of hydroxy-terminated rubber, e.g., hydroxy-terminated polybutadiene, which is then reacted with a polyether triol and an isocyanate component in the presence of a mercury catalyst, carbon black, a low oil absorbency silica and a suspending agent therefore, lecithin, and a molecular sieve desiccant. U.S. Pat. No. 4,929,469 discloses a UV-curable surface protective coating comprising a liquid diene rubber of molecular weight from 1,000 to 10,000 and having one or more hydroxyl groups. Additionally, a diisocyanate component, a diol, and an ethylenically unsaturated monomer having at least one hydroxy group must be present and the composition is then screen printed and cured by UV radiation to form a coating which is easily peeled off of a printed circuit board after plating and soldering. Therefore, the coating systems disclosed in these references are based on polyurethane chemistry and not rubber vulcanization.
The highly chemically resistant rubber covering disclosed herein is provided by adding at least one powdered substance, sometimes known as an active filler, to the liquid rubber thereby creating a two phase structure or a composite. The phase comprising the filler is believed to be transformed into a new phase as a result of the interaction of the filler with an aggressive medium, such as water, aqueous acid or aqueous alkali, which penetrates the LEM rubber comprising the filler. This new filler phase is believed to comprise a high strength hydrate complex, as will be discussed in detail below. As aggressive medium penetration causes, e.g., hydrate complex formation, the volume of the filler becomes greater than the volume of the initial filler. As a result of filler particle volume growth, the free volume of the composite decreases.
Moreover, in order to further improve the properties and decrease cost, thereby expanding the range of use of these composites, organic and inorganic fillers and aggregates may also be incorporated into the LEM compositions of the invention.
Furthermore, the LEM compositions of the present invention are ideally suited for use as binders, such as are employed in pharmaceutical formulations and in rubber concrete. The conventional binders used in polymer concrete are unsaturated polyesters, epoxy resins and, to some extent, furan and acrylic resins. However, the future use of unsaturated polyesters, which comprise volatile styrene monomer, is likely to be strongly restricted by laws limiting styrene emissions. Furthermore, high strength epoxy resin-based polymer concretes are very costly, therefore, their use is limited to relatively cost-insensitive applications such as cavitation resistant materials for offshore structures, monolithic flooring and to applications in the machine-tool making industry.
Therefore, advanced rubber concretes comprising the liquid ebonite mixtures of the present invention as binders are very useful because they avoid these disadvantages of the polymers conventionally used in concrete formulations. Moreover, hydrolysis resistant LEM binders make possible the preparation of rubber concretes with high acid and alkali resistance, good toughness and excellent adhesion to the steel reinforcement typically found in reinforced structural concrete.